ESTRO 2021 Abstract Book

S123

ESTRO 2021

were 24% and 5%, respectively. Observed Grade≥2 AET was significantly lower with IMPT compared with the calculated NTCP for VMAT (29% vs 42%; p=0.014). So far, only one patient (4.8%) experienced RP, however, only 21 patients had ≥6 months of follow-up. Follow-up will continuously be updated and all new patients treated with IMPT will be added to this dynamic database. Conclusion Based on the model based selection of (N)SCLC patients for proton radiotherapy, IMPT may be beneficial for a substantial part of these patients in terms of reduction in predicted toxicity. The first results on toxicity seem promising.

Proffered papers: Proffered papers 11: Detectors and dosimetry

OC-0194 Monte Carlo determination of magnetic field correction factors kB,Q for dosimetry in 1.5T MR- Linacs V. Margaroni 1 , E.P. Pappas 1 , A. Episkopakis 1,2 , P. Karaiskos 1,3 1 National and Kapodistrian University of Athens, Medical School - Medical Physics Laboratory, Athens, Greece; 2 Elekta , Ltd, Crawley, United Kingdom; 3 Hygeia Hospital, Medical Physics Department, Marousi, Greece Purpose or Objective Dosimetry in 1.5T MR-linacs is challenging due to the presence of the strong magnetic field, B, which impacts on the trajectory of secondary electrons. Current codes-of-practice and formalisms for dosimetry in conventional linacs need to be extended in order to account for corresponding changes in the detector

readings. In response to this, an additional chamber- and orientation-specific correction factor , k B,Q , (hereinafter, CF) has been recently introduced, to correct for detector readings under the reference field Q.